Question

Functions are the only things in JavaScript that create a new scope. So if we want our modules to have their own scope, we will have to base them on functions.

Consider this trivial module for associating names with day-of-the-week numbers, as returned by a Date object’s getDay method:

var names = ["Sunday", "Monday", "Tuesday", "Wednesday",
             "Thursday", "Friday", "Saturday"];
function dayName(number) {
  return names[number];
}

console.log(dayName(1));
// → Monday

The dayName function is part of the module’s interface, but the names variable is not. We would prefer not to spill it into the global scope.

We can do this:

var dayName = function() {
  var names = ["Sunday", "Monday", "Tuesday", "Wednesday",
               "Thursday", "Friday", "Saturday"];
  return function(number) {
    return names[number];
  };
}();

console.log(dayName(3));
// → Wednesday

Now names is a local variable in an (unnamed) function. This function is created and immediately called, and its return value (the actual dayName function) is stored in a variable. We could have pages and pages of code in this function, with 100 local variables, and they would all be internal to our module—visible to the module itself but not to outside code.

We can use a similar pattern to isolate code from the outside world entirely. The following module logs a value to the console but does not actually provide any values for other modules to use:

(function() {
  function square(x) { return x * x; }
  var hundred = 100;

  console.log(square(hundred));
})();
// → 10000

This code simply outputs the square of 100, but in the real world it could be a module that adds a method to some prototype or sets up a widget on a web page. It is wrapped in a function to prevent the variables it uses internally from polluting the global scope.

Why did we wrap the namespace function in a pair of parentheses? This has to do with a quirk in JavaScript’s syntax. If an expression starts with the keyword function , it is a function expression. However, if a statement starts with function , it is a function declaration, which requires a name and, not being an expression, cannot be called by writing parentheses after it. You can think of the extra wrapping parentheses as a trick to force the function to be interpreted as an expression.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.